1. Field of Invention
This invention relates generally to the field of industrial pipeline pigging and more specifically to a device that can simultaneously control the flow path as a fluid enters or leaves a piggable pipeline, as well as the movement of pigs within the piggable pipeline.
2. Description of Prior Art
Pigging is used in industrial applications to clean, scrape or purge piping systems. Components used in these applications are engineered to launch, catch and control the movement of special in-line clearing devices called pigs. The pigs are moved through the inside of the piping systems using propellants such as compressed air, inert gases, or pumped fluids. Pigs can resemble spheres, cylinders, or other shapes and can be equipped with a variety of features to enhance their cleaning function; such as wire brushes, abrasive materials, scrapers, or squeegee-like blades. As a pig is propelled through the pipeline, its surface is used to physically clean or scrape the inside of the pipeline to remove unwanted debris or sediment, or to ensure that any material present in the pipeline is “pushed” downstream by the pig. Cleaning efficiencies can vary with pig design, pipeline design, and the speed at which the pig is propelled through the pipeline.
An important application of pigging technology can be found where a common distribution pipeline is used to supply multiple destinations. In these systems, various fluid products may be produced in one or more mixing tanks and then pumped via a common distribution pipeline to predetermined destination tanks. Points along the distribution pipeline where fluid flows into, or out of, the pipeline are called branch points. For a given transfer, a fluid would be pumped into the distribution pipeline at a predetermined branch point and then discharged downstream from a second predetermined branch point. The pigging operation should ensure that sections of the pipeline not needed for the transfer, upstream of the entry point and downstream of the discharge point, are blocked off and that all of the fluid pumped into the line is efficiently discharged from the line. Once the pigging operation is complete, any fluid remaining in the pipeline is a potential source of cross contamination for subsequent transfers.
Hence, the device or devices used at each branch point must control fluid flow into and/or out of the distribution pipeline, create upstream and downstream seals in the pipeline during the transfer, allow a pig to traverse the section of pipeline used for the transfer and provide a means to stop the pig at the discharge branch point with minimum fluid retention. Common devices used to meet this requirement fall into two categories: (1) Piggable Tee and Line Block and (2) Branch Valve and Pig Stop.
(1) Piggable Tee and Line Block Category:
In this category piggable tees and piggable ball valves are installed in the distribution pipeline at branch points and block valves are installed in the branch pipelines as close as possible to the distribution pipeline. The piggable tees in the distribution pipeline are constructed with internal guides to prevent a pig from following the fluid flow from the distribution pipeline into a branch pipeline. The ball valves in the distribution pipeline are used to control pig movement and provide seals downstream of the transfer operation. The block valves in the branch pipelines are operated to control flow to and from the distribution pipeline. When a product transfer is complete a pig is launched toward the branch discharge point to expel fluid from the distribution pipeline into the branch pipeline. The Piggable ball valve in the distribution pipeline creates a seal downstream of the branch point to stop the pig and to divert fluid flow into the branch pipeline. The piggable tee allows the fluid to flow into the branch pipeline and prevents the pig from following. The piggable tees used in this approach have the disadvantage of forming pockets at each branch point that trap and retain fluid after each transfer operation. This retained fluid can be a potential source of cross contamination for subsequent transfers. The amount of retention can be minimized through strategic orientation of the piggable tees with respect to the distribution pipeline, but it cannot be eliminated. Three devices are required at each branch point to implement this approach; (1) a piggable tee in the piggable transfer line, (2) a piggable valve in the piggable transfer line, and (3) a block valve in the branch pipeline.
(2) Branch Valve and Pig Stop Category (There are at least two devices commercially available that fall into this category.)
In this category a single device incorporates the functions of the piggable tee and the block valve in the branch pipeline, and also provides a retractable pig stop in the distribution pipeline. These devices utilize two pigs in the distribution pipeline, one upstream and one downstream of the transfer operation. Retractable pig stops (steel rods of suitable size or other shapes) are inserted into the pipeline to control pig movement. After a product transfer is complete, a pig stop is inserted at the branch discharge point and the upstream pig is launched to “push” remaining fluid toward the branch discharge point. Any fluid in the pipe line upstream of the branch discharge point is expelled at the branch discharge point as the pig comes to rest against the upstream side of the pig stop. The downstream pig is launched toward the pig stop from a point downstream of the branch discharge point. Any fluid downstream of the branch discharge point is similarly expelled as the pig comes to rest against the downstream side of the pig stop. In this way, only the small quantity of fluid trapped around at the pig stop between the opposing pigs is left in the pipeline.
These devices have the disadvantage of requiring the use of two pigs in the distribution pipeline, which complicates the pigging operation, can increase the cost and complexity of the pig launchers, and increases the cost of system programming (if automated). Also as the pigs wear, their ability to form positive seals in the piggable transfer line is degraded. Worn pigs can allow the propellant to leak around the pigs during the transfer operation and thus become entrained in the fluid being transferred.
Industrial applications of piggable systems often include automation of system components. While the two categories discussed above incorporate components that are easily automated, they have the disadvantage of requiring multiple components. It can be appreciated that cost of actuators, limit switches, signal wiring, programming time, and field installation will increase with the number of components that must be automated. And since use of the present invention will facilitate an approach requiring only one component per branch point, the cost of system automation will be significantly reduced.